[PATCH v4 30/41] tracing/events: Add __vstring() and __assign_vstr() helper macros

[Jim Cromie]

From: "Steven Rostedt (Google)" <rostedt at goodmis.org>

Steve's patch, carried til upstream.

There's several places that open code the following logic:

  TP_STRUCT__entry(__dynamic_array(char, msg, MSG_MAX)),
  TP_fast_assign(vsnprintf(__get_str(msg), MSG_MAX, vaf->fmt, *vaf->va);)

To load a string created by variable array va_list.

The main issue with this approach is that "MSG_MAX" usage in the
__dynamic_array() portion. That actually just reserves the MSG_MAX in the
event, and even wastes space because there's dynamic meta data also saved
in the event to denote the offset and size of the dynamic array. It would
have been better to just use a static __array() field.

Instead, create __vstring() and __assign_vstr() that work like __string
and __assign_str() but instead of taking a destination string to copy,
take a format string and a va_list pointer and fill in the values.

It uses the helper:

 #define __trace_event_vstr_len(fmt, va)	\
 ({						\
	va_list __ap;				\
	int __ret;				\
						\
	va_copy(__ap, *(va));			\
	__ret = vsnprintf(NULL, 0, fmt, __ap);	\
	va_end(__ap);				\
						\
	min(__ret, TRACE_EVENT_STR_MAX);	\
 })

To figure out the length to store the string. It may be slightly slower as
it needs to run the vsnprintf() twice, but it now saves space on the ring
buffer.

Link: https://lkml.kernel.org/r/20220705224749.053570613@goodmis.org

Cc: Dennis Dalessandro <dennis.dalessandro at cornelisnetworks.com>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Jason Gunthorpe <jgg at ziepe.ca>
Cc: Leon Romanovsky <leon at kernel.org>
Cc: Kalle Valo <kvalo at kernel.org>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: Eric Dumazet <edumazet at google.com>
Cc: Jakub Kicinski <kuba at kernel.org>
Cc: Paolo Abeni <pabeni at redhat.com>
Cc: Arend van Spriel <aspriel at gmail.com>
Cc: Franky Lin <franky.lin at broadcom.com>
Cc: Hante Meuleman <hante.meuleman at broadcom.com>
Cc: Gregory Greenman <gregory.greenman at intel.com>
Cc: Peter Chen <peter.chen at kernel.org>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Mathias Nyman <mathias.nyman at intel.com>
Cc: Chunfeng Yun <chunfeng.yun at mediatek.com>
Cc: Bin Liu <b-liu at ti.com>
Cc: Marek Lindner <mareklindner at neomailbox.ch>
Cc: Simon Wunderlich <sw at simonwunderlich.de>
Cc: Antonio Quartulli <a at unstable.cc>
Cc: Sven Eckelmann <sven at narfation.org>
Cc: Johannes Berg <johannes at sipsolutions.net>
Cc: Jim Cromie <jim.cromie at gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt at goodmis.org>
---
 include/linux/trace_events.h                 | 18 ++++++++++++++++++
 include/trace/stages/stage1_struct_define.h  |  3 +++
 include/trace/stages/stage2_data_offsets.h   |  3 +++
 include/trace/stages/stage4_event_fields.h   |  3 +++
 include/trace/stages/stage5_get_offsets.h    |  4 ++++
 include/trace/stages/stage6_event_callback.h |  7 +++++++
 6 files changed, 38 insertions(+)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index e6e95a9f07a5..b18759a673c6 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -916,6 +916,24 @@ perf_trace_buf_submit(void *raw_data, int size, int rctx, u16 type,
 
 #endif
 
+#define TRACE_EVENT_STR_MAX	512
+
+/*
+ * gcc warns that you can not use a va_list in an inlined
+ * function. But lets me make it into a macro :-/
+ */
+#define __trace_event_vstr_len(fmt, va)			\
+({							\
+	va_list __ap;					\
+	int __ret;					\
+							\
+	va_copy(__ap, *(va));				\
+	__ret = vsnprintf(NULL, 0, fmt, __ap) + 1;	\
+	va_end(__ap);					\
+							\
+	min(__ret, TRACE_EVENT_STR_MAX);		\
+})
+
 #endif /* _LINUX_TRACE_EVENT_H */
 
 /*
diff --git a/include/trace/stages/stage1_struct_define.h b/include/trace/stages/stage1_struct_define.h
index a16783419687..1b7bab60434c 100644
--- a/include/trace/stages/stage1_struct_define.h
+++ b/include/trace/stages/stage1_struct_define.h
@@ -26,6 +26,9 @@
 #undef __string_len
 #define __string_len(item, src, len) __dynamic_array(char, item, -1)
 
+#undef __vstring
+#define __vstring(item, fmt, ap) __dynamic_array(char, item, -1)
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(char, item, -1)
 
diff --git a/include/trace/stages/stage2_data_offsets.h b/include/trace/stages/stage2_data_offsets.h
index 42fd1e8813ec..1b7a8f764fdd 100644
--- a/include/trace/stages/stage2_data_offsets.h
+++ b/include/trace/stages/stage2_data_offsets.h
@@ -32,6 +32,9 @@
 #undef __string_len
 #define __string_len(item, src, len) __dynamic_array(char, item, -1)
 
+#undef __vstring
+#define __vstring(item, fmt, ap) __dynamic_array(char, item, -1)
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)
 
diff --git a/include/trace/stages/stage4_event_fields.h b/include/trace/stages/stage4_event_fields.h
index e80cdc397a43..c3790ec7a453 100644
--- a/include/trace/stages/stage4_event_fields.h
+++ b/include/trace/stages/stage4_event_fields.h
@@ -38,6 +38,9 @@
 #undef __string_len
 #define __string_len(item, src, len) __dynamic_array(char, item, -1)
 
+#undef __vstring
+#define __vstring(item, fmt, ap) __dynamic_array(char, item, -1)
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)
 
diff --git a/include/trace/stages/stage5_get_offsets.h b/include/trace/stages/stage5_get_offsets.h
index 7ee5931300e6..fba4c24ed9e6 100644
--- a/include/trace/stages/stage5_get_offsets.h
+++ b/include/trace/stages/stage5_get_offsets.h
@@ -39,6 +39,10 @@
 #undef __string_len
 #define __string_len(item, src, len) __dynamic_array(char, item, (len) + 1)
 
+#undef __vstring
+#define __vstring(item, fmt, ap) __dynamic_array(char, item,		\
+		      __trace_event_vstr_len(fmt, ap))
+
 #undef __rel_dynamic_array
 #define __rel_dynamic_array(type, item, len)				\
 	__item_length = (len) * sizeof(type);				\
diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
index e1724f73594b..0f51f6b3ab70 100644
--- a/include/trace/stages/stage6_event_callback.h
+++ b/include/trace/stages/stage6_event_callback.h
@@ -24,6 +24,9 @@
 #undef __string_len
 #define __string_len(item, src, len) __dynamic_array(char, item, -1)
 
+#undef __vstring
+#define __vstring(item, fmt, ap) __dynamic_array(char, item, -1)
+
 #undef __assign_str
 #define __assign_str(dst, src)						\
 	strcpy(__get_str(dst), (src) ? (const char *)(src) : "(null)");
@@ -35,6 +38,10 @@
 		__get_str(dst)[len] = '\0';				\
 	} while(0)
 
+#undef __assign_vstr
+#define __assign_vstr(dst, fmt, va)					\
+	vsnprintf(__get_str(dst), TRACE_EVENT_STR_MAX, fmt, *(va))
+
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)
 
-- 
2.36.1